Cryogenic refrigerator

ABSTRACT

A cryogenic refrigerator includes a cylinder; a displacer configured to reciprocate in the cylinder; a driving unit configured to drive a rotational shaft having an axial direction oriented in a direction of reciprocating movement of the displacer; and a movement conversion unit configured to convert rotational movement of the rotational shaft into the reciprocating movement of the displacer. The movement conversion unit is a screw unit including a male thread and a female thread one of which is a part of the displacer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priority of Japanese Patent Application No. 2012-039245 filed on Feb. 24, 2012, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An aspect of this disclosure relates to a cryogenic refrigerator.

2. Description of the Related Art

Japanese Laid-Open Patent Publication No. 3-84368, for example, discloses a cryogenic refrigerator where valves are opened and closed while causing a displacer to reciprocate in a cylinder. In this cryogenic refrigerator, “coldness” is generated by causing a refrigerant gas in an expansion space to expand via a clearance between the displacer and the cylinder, the “coldness” is transferred to a cooling stage on the periphery of the clearance and the expansion space by heat exchange, and as a result, an object is frozen.

However, with the related-art configuration where a complex mechanism such as a Scotch yoke mechanism or a ball screw mechanism is used to reciprocate the displacer, the number of components and the outside dimensions of a cryogenic refrigerator tend to become large. Also, when a ball screw mechanism is used, it is necessary to use an axial gap motor or a motor with a hollow rotor and therefore the costs of a cryogenic refrigerator may increase.

SUMMARY OF THE INVENTION

In an aspect of this disclosure, there is provided a cryogenic refrigerator that includes a cylinder; a displacer configured to reciprocate in the cylinder; a driving unit configured to drive a rotational shaft having an axial direction oriented in a direction of reciprocating movement of the displacer; and a movement conversion unit configured to convert rotational movement of the rotational shaft into the reciprocating movement of the displacer. The movement conversion unit is a screw unit including a male thread and a female thread one of which is a part of the displacer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary configuration of a cryogenic refrigerator according to a first embodiment;

FIG. 2 is a schematic diagram illustrating an exemplary configuration of a live load packing of the cryogenic refrigerator of the first embodiment; and

FIG. 3 is a schematic diagram illustrating an exemplary configuration of a cryogenic refrigerator according to a second embodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention are described below with reference to the accompanying drawings.

First Embodiment

A cryogenic refrigerator 1 of a first embodiment may be implemented, for example, as a Gifford-McMahon (GM) refrigerator that uses a helium gas as the refrigerant gas. As illustrated in FIG. 1, the cryogenic refrigerator 1 includes a cylinder 2, a displacer 3, a motor shaft (rotational shaft) 4, a motor (driving unit) 5, a sleeve 6, a live load packing 7, a housing 8, a valve plate 9, a valve body 10, and a gas path 11.

The cylinder 2 is a closed-end cylinder that encloses the displacer 3. The upper end (in FIG. 1) of the cylinder 2 is hermetically closed by the housing 8. An expansion space 12 is formed by the displacer 3 and the cylinder 2 in the lower part of FIG. 1. Although not illustrated in FIG. 1, the cylinder 2 includes a cooling stage that is disposed adjacent to and surrounds the expansion space 12. The displacer 3 faces the cooling stage across a clearance. The cooling stage may be composed of, for example, copper, aluminum, or stainless steel.

The displacer 3 is housed in the cylinder 2 so as to be able to reciprocate in the longitudinal direction or the axial direction of the motor 5. For the cylinder 2, a material such as stainless steel may be used taking into account strength, thermal conductivity, and helium blocking capability.

The motor 5 causes the displacer 3 to reciprocate and is disposed at a hot end of the cylinder 2 such that the motor shaft 4 is oriented in the axial direction of the displacer 3 and the cylinder 2 (or in the direction of the reciprocating movement of the displacer 3). The outer circumferential surface of the displacer 3 is shaped like a cylinder, and the displacer is filled with a cold storage medium. The internal volume of the displacer 3 constitutes a regenerator. An upper flow smoother for smoothing the flow of the helium gas is provided at the upper end of the regenerator, i.e., the end closer to the ambient temperature chamber, and a lower flow smoother is provided at the lower end of the regenerator.

An upper opening is formed at the hot end of the displacer 3. The refrigerant gas flows from the ambient temperature chamber, which is located above the displacer 3 in FIG. 1, through the upper opening into the displacer 3. The ambient temperature chamber is a space formed by the cylinder 2 and the hot end of the displacer 3, and the volume of the ambient temperature chamber changes as the displacer 3 reciprocates.

An intake and exhaust system implemented by a rotary valve composed of the valve plate 9 and the valve body 10 is provided in the housing 8 that is disposed, in the axial direction, between the motor 5 and the cylinder 2. A high-pressure flexible pipe HF connected to the high-pressure side of a compressor (not shown) and a low-pressure flexible pipe LF connected to the low-pressure side of the compressor are connected to one end of the intake and exhaust system. Meanwhile, the gas path 11 implementing a supply and exhaust pipe and leading to the ambient temperature chamber is connected to the other end of the intake and exhaust system. The live load packing 7 is disposed between the cylinder 2 and a part of the displacer 3 near the hot end.

As illustrated in FIG. 2, the live load packing 7 includes a chevron packing 7 a (intermediate layer, triangle-shaped), upper and lower packings 7 b (sandwich layers), a ground 7 c, and bolts 7 d. The chevron packing 7 a includes a pair of string- or fiber-like elements. The elements have a cross-section shaped like a right triangle and their sloping surfaces are in contact with each other. The elements contain a lubricant.

The upper and lower packings 7 b sandwich the chevron packing 7 a in the axial direction. The lower packing 7 b is disposed in a recess formed in the cylinder 2 adjacent to the hot end of a male thread 3 a of the displacer 3. The chevron packing 7 a is disposed above the lower packing 7 b, the upper packing 7 b is disposed above the chevron packing 7 a, and the ground 7 c is disposed above the upper packing 7 b as a spacer.

A wall part of the housing 8 is disposed above the ground 7 c. Female screw holes are formed through the wall part of the housing 8 c in the vertical direction in FIG. 2. The bolts 7 d are screwed into the female screw holes. The end of the male thread part (male thread end) of each bolt 7 d is in contact with the upper surface of the ground 7 c. When the bolts 7 d are screwed into the female screw holes and the male tread ends are moved downward, the force of the upper and lower packings 7 b sandwiching the chevron packing 7 a is increased. As a result, the sloping surfaces of the elements constituting the chevron packing 7 a are pressed with a stronger force, and the inner one of the elements is pressed with a stronger force against the outer circumferential surface of the displacer 3 and thereby seals the displacer 3 while supplying the lubricant.

A lower opening for introducing the refrigerant gas via the clearance into the expansion space 12 is formed at a cold end of the displacer 3. The expansion space 12 is a space formed by the cylinder 2 and the displacer 3, and the volume of the expansion space 12 changes as the displacer 3 reciprocates. The cooling stage is disposed on the outer circumferential surface of the cylinder 2 at a position corresponding to the expansion space 12 and is thermally connected to an object to be cooled. The cooling stage is cooled by the refrigerant gas passing through the clearance.

For the displacer 3, a material such as Bakelite (fabric-filled phenolic material) may be used taking into account specific gravity, abrasion resistance, strength, and thermal conductivity. As the cold storage medium, for example, a wire mesh may be used. The male thread 3 a is formed on a part of the outer circumferential surface of the displacer 3 closer to the hot end, and a female thread 2 a that engages the male thread 3 a is formed on the inner circumferential surface of the cylinder 2. A fitting hole is formed in the hot end surface of the displacer 3. The sleeve 6 is shaped like a cylinder and is fitted into the fitting hole, and the movement of the sleeve 6 relative to the displacer 3 in the circumferential direction and the axial direction is thereby prevented.

The outer circumferential surface of the sleeve 6 is, for example, serrated. An inner hole having an inner circumferential surface corresponding to the sleeve 6 is formed in the valve plate 9, and the inner circumferential surface of the inner hole is also serrated to correspond to the serration on the outer circumferential surface of the sleeve 6. A cylindrical inner hole having a diameter greater than the maximum diameter of the sleeve 6 is formed in the valve body 10. Thus, the movement of the sleeve 6 relative to the valve body 10 is not prevented in the circumferential direction and the axial direction. Meanwhile, the movement of the sleeve 6 relative to the valve plate 9 is not prevented in the axial direction but is prevented in the circumferential direction. In other words, when the motor 5 rotates in the forward direction or the reverse direction, the valve plate 9 is rotated via the sleeve 6 relative to the valve body 10 and thereby functions as a supply valve and a return valve during the operations of the cryogenic refrigerator 1.

The inner circumferential surface of the sleeve 6 is also serrated, and the outer circumferential surface of the motor shaft 4 is serrated to correspond to the serration on the inner circumferential surface of the sleeve 6. Accordingly, the movement of the sleeve 6 relative to the motor shaft 4 is not prevented in the axial direction but is prevented in the circumferential direction.

The motor 5 is implemented by, for example, a three-phase AC synchronous motor, and is driven (rotated) in the forward and reverse directions by an inverter (not shown). When the motor 5 is driven in the forward direction, the motor shaft 4 rotates in the forward direction and as a result, the sleeve 6 and the displacer 3 drivably coupled to the motor shaft 4 are rotated in the forward direction. When rotated in the forward direction, the displacer 3 is caused to move from the bottom dead point (upper position in FIG. 1) to the top dead point (lower position in FIG. 1) by a screw unit composed of the male thread 3 a and the female thread 2 a. In this case, downward movement of the sleeve is absorbed by the relative movement between the sleeve 6 and the motor shaft 4. Meanwhile, when the motor 5 is driven in the reverse direction, the displacer 3 is caused to move from the top dead point to the bottom dead point.

Next, exemplary operations of the cryogenic refrigerator 1 are described. At a timing in a refrigerant gas supplying step, the displacer 3 is located at the top dead point (lower position in FIG. 1) in the cylinder 2. At the same timing or at a slightly different timing, the supply valve implemented by the rotary valve is opened. As a result, high-pressure helium gas is supplied via the supply valve through the gas path 11 (supply and exhaust pipe) into the cylinder 2. Then, the high-pressure helium gas flows through the upper opening at the upper end of the displacer 3 into the regenerator in the displacer 3. The high-pressure helium gas flowing into the regenerator is cooled by the cool storage medium, flows through the lower opening at the lower end of the displacer 3 and the clearance, and is supplied into the expansion space 12.

When the expansion space 12 is filled with the high-pressure helium gas, the supply valve is closed. At this timing, the displacer 3 is located at the bottom dead point (upper position in FIG. 1) in the cylinder 2. At the same timing or at a slightly different timing, the return valve implemented by the rotary valve is opened, the pressure of the helium gas in the expansion space 12 is reduced and the helium gas expands. Due to the expansion, the helium gas in the expansion space 12 is cooled and absorbs the heat of the cooling stage via the clearance.

Then, the displacer 3 moves toward the top dead point (lower position in FIG. 1) and the volume of the expansion space 12 decreases. As a result, the helium gas in the expansion space 12 returns via the clearance, the lower opening, the regenerator, and the upper opening to the intake side of the compressor. When the helium gas (refrigerant gas) flows through the regenerator, the cold storage medium is cooled by the helium gas. The above process is referred to as a “cooling cycle”, and the cryogenic refrigerator 1 repeats the cooling cycle to cool the cooling stage.

In the cryogenic refrigerator 1 of the first embodiment, the driving force of the motor 5 is converted into the reciprocating movement of the displacer 3 between the bottom dead point and the top dead point by the screw unit composed of the male thread 3 a and the female thread 2 a. Thus, the configuration of the first embodiment makes it possible to eliminate the need to use a Scotch yoke mechanism or a ball screw mechanism. Unlike a Scotch yoke mechanism, the configuration of the first embodiment makes it possible to orient the axis of the motor 5 in the vertical direction. This in turn makes it possible to prevent excessive thrust load on the motor shaft 4 of the motor 5. Also, unlike a ball screw mechanism, the configuration of the first embodiment makes it possible to use a normal motor as the motor 5 and thereby makes it possible to reduce the costs of the cryogenic refrigerator 1. Further, since the screw unit, i.e., a movement conversion unit, is implemented by the male thread 3 a and the female thread 2 a that are disposed to overlap the displacer 3 in the axial direction, it is possible to reduce the size in the axial direction and the number of parts of the cryogenic refrigerator 1 compared with the related-art configuration.

The screw unit is composed of the male thread 3 a formed on the outer circumferential surface of the displacer 3 and the female thread 2 a formed on the inner circumferential surface of the cylinder 2, and therefore the screw movement is achieved by the male thread 3 a and the female thread 2 a that are in close contact with each other. This configuration makes it possible to reduce the leakage of helium gas flowing through the clearance from the hot side to the cold side. Also with this configuration, since even leaked helium gas flows helically, it is possible to increase the contact time of the helium gas with the outer circumferential surface of the displacer 3 and thereby increase the flow-path resistance in the clearance. In other words, this configuration makes it possible to reduce penetration of heat into the cooling stage due to the leakage of helium gas.

In the configuration of the first embodiment, the displacer 3 rotates in the circumferential direction with respect to the cylinder 2. Even in this case, the live load packing 7 configured as described above can effectively seal the clearance that is a gap in the radial direction between the displacer 3 and the cylinder 2. Particularly, the live load packing 7 can increase the lubricity and reduce the frictional resistance of the displacer 3 that rotates in the circumferential direction with respect to the cylinder 2. Alternatively, the live load packing 7 may be replaced with an O-ring or a slipper seal.

Second Embodiment

In the first embodiment, the screw unit is formed at the outer side of the displacer 3. Alternatively, the screw unit may be formed at the inner side of the displacer 3. FIG. 3 is a schematic diagram illustrating an exemplary configuration of a cryogenic refrigerator 21 of a second embodiment. The same reference numbers are used for components corresponding to those in the first embodiment, and descriptions of those components are omitted. Here, differences between the first embodiment and the second embodiment are mainly described.

The cryogenic refrigerator 21 of the second embodiment may also be implemented as a Gifford-McMahon (GM) refrigerator. As illustrated in FIG. 3, the cryogenic refrigerator 21 includes a cylinder 22, a displacer 23, a motor shaft (rotational shaft) 4, a motor (driving unit) 5, a sleeve 6, a slipper seal 27, a housing 8, a valve plate 9, a valve body 10, and a gas path 11.

The cylinder 22 is a closed-end cylinder that encloses the displacer 23. The upper end of the cylinder 22 is hermetically closed by the housing 8. An expansion space is formed by the displacer 23 and the cylinder 22 in the lower part of FIG. 3. Although not illustrated in FIG. 3, the cylinder 22 includes a cooling stage that is disposed adjacent to and surrounds the expansion space. The displacer 23 faces the cooling stage across a clearance. The cooling stage may be composed of, for example, copper, aluminum, or stainless steel.

The displacer 23 is housed in the cylinder 22 so as to be able to reciprocate in the longitudinal direction or the axial direction of the motor 5. For the cylinder 22, a material such as stainless steel may be used taking into account strength, thermal conductivity, and helium blocking capability.

The motor 5 causes the displacer 23 to reciprocate and is disposed at a hot end of the cylinder 22 such that the motor shaft 4 is oriented in the axial direction of the displacer 23 and the cylinder 22. The outer circumferential surface of the displacer 23 is shaped like a cylinder, and the displacer 23 is filled with a cold storage medium. The internal volume of the displacer 23 constitutes a regenerator. An upper flow smoother for smoothing the flow of the helium gas is provided at the upper end of the regenerator, i.e., the end closer to the ambient temperature chamber, and a lower flow smoother is provided at the lower end of the regenerator.

An upper opening is formed at the hot end of the displacer 23. The refrigerant gas flows from the ambient temperature chamber, which is located above the displacer 23 in FIG. 3, through the upper opening into the displacer 23. The ambient temperature chamber is a space formed by the cylinder 22 and the hot end of the displacer 23, and the volume of the ambient temperature chamber changes as the displacer 23 reciprocates.

In the housing 8 disposed between the motor 5 and the cylinder 22 in the axial direction, a high-pressure flexible pipe HF connected to the high-pressure side of a compressor (not shown), a low-pressure flexible pipe LF connected to the low-pressure side of the compressor, and an intake and exhaust system implemented by a rotary valve composed of the valve plate 9 and the valve body 10 are provided. The gas path 11 implementing a supply and exhaust pipe of the intake and exhaust system is connected to the ambient temperature chamber.

A lower opening for introducing the refrigerant gas via the clearance into the expansion space is formed at a cold end of the displacer 23. The expansion space is formed by the cylinder 22 and the displacer 23, and the volume of the expansion space changes as the displacer 23 reciprocates. The cooling stage is disposed on the outer circumferential surface of the cylinder 22 at a position corresponding to the expansion space 12 and is thermally connected to an object to be cooled. The cooling stage is cooled by the refrigerant gas passing through the clearance.

For the displacer 23, a material such as Bakelite (fabric-filled phenolic material) may be used taking into account specific gravity, abrasion resistance, strength, and thermal conductivity. As the cold storage medium, for example, a wire mesh may be used. An upper cup 23 b shaped like a disk is provided at the hot end of the displacer 23. The slipper seal 27 is disposed on the outer circumferential surface of the displacer 23 at a position closer than the upper cup 23 b to the cold end of the displacer 23.

In the center of a hot-end surface of the displacer 23, a hole (or recess) extending toward a cold-end surface of the displacer 23 is formed. A female thread 23 a is formed on the inner circumferential surface of the hole. A male thread 26 a corresponding to the female thread 23 a is formed on the outer circumferential surface of a sleeve (cylindrical part) 26.

A part of the outer surface of the sleeve 26, which is positioned higher than the male thread 26 a in FIG. 3, is serrated. An inner hole having an inner surface corresponding to the sleeve 26 is formed in the valve plate 9, and the inner surface of the inner hole is also serrated to correspond to the serration on the outer circumferential surface of the sleeve 6. A cylindrical inner hole having a diameter greater than the maximum diameter of the sleeve 26 is formed in the valve body 10.

Thus, the movement of the sleeve 26 relative to the valve body 10 is not prevented in the circumferential direction and the axial direction. Meanwhile, the movement of the sleeve 26 relative to the valve plate 9 is not prevented in the axial direction but is prevented in the circumferential direction. In other words, when the motor 5 rotates in the forward direction or the reverse direction, the valve plate 9 is rotated via the sleeve 26 relative to the valve body 10 and thereby functions as a supply valve and a return valve during the operations of the cryogenic refrigerator 21.

The inner circumferential surface of the sleeve 26 is also serrated, and the outer circumferential surface of the motor shaft 4 is serrated to correspond to the serration on the inner circumferential surface of the sleeve 6. Accordingly, the movement of the sleeve 26 relative to the motor shaft 4 is not prevented in the axial direction but is prevented in the circumferential direction.

The motor 5 is implemented by, for example, a three-phase AC synchronous motor, and is driven (rotated) in the forward and reverse directions by an inverter (not shown). When the motor 5 is driven in the forward direction, the motor shaft 4 rotates in the forward direction and as a result, the sleeve 26 drivably coupled to the motor shaft 4 is rotated in the forward direction. When the sleeve 26 is rotated in the forward direction, the displacer 23 is caused to move from the bottom dead point (upper position in FIG. 3) to the top dead point (lower position in FIG. 3) by a screw unit composed of the male thread 26 a and the female thread 23 a. Here, it is assumed that the displacer 23 has a sufficient weight with respect to the circumferential frictional force of the screw unit so that the displacer 23 is not rotated by the rotation of the sleeve 26 in the circumferential direction relative to the cylinder 22. Also, a mechanism such as a key may be provided to more reliably prevent the rotation of the displacer 23 relative to the cylinder 22.

The downward movement of the sleeve 26 is absorbed by the relative movement between the sleeve 26 and the motor shaft 4. Meanwhile, when the motor 5 is driven in the reverse direction, the displacer 23 is caused to move from the top dead point to the bottom dead point. Other operations of the cryogenic refrigerator 21 are substantially the same as those of the cryogenic refrigerator 1 of the first embodiment.

In the cryogenic refrigerator 21 of the second embodiment, the driving force of the motor 5 is converted into the reciprocating movement of the displacer 23 between the bottom dead point and the top dead point by the screw unit composed of the male thread 26 a and the female thread 23 a. Thus, the configuration of the second embodiment also makes it possible to eliminate the need to use a Scotch yoke mechanism or a ball screw mechanism. Unlike a Scotch yoke mechanism, the configuration of the second embodiment makes it possible to orient the axis of the motor 5 in the vertical direction. This in turn makes it possible to prevent excessive thrust load on the motor shaft 4 of the motor 5. Also, unlike a ball screw mechanism, the configuration of the second embodiment makes it possible to use a normal motor as the motor 5 and thereby makes it possible to reduce the costs of the cryogenic refrigerator 21. Further, since the screw unit, i.e., a movement conversion unit, is implemented by the male thread 26 a and the female thread 23 a that are disposed to overlap the displacer 23 in the axial direction, it is possible to reduce the size in the axial direction and the number of parts of the cryogenic refrigerator 21 compared with the related-art configuration.

The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

For example, in the above embodiments, it is assumed that the number of stages of a cryogenic refrigerator is one. However, the present invention may also be applied to a cryogenic refrigerator having any number of stages. When a cryogenic refrigerator includes two or more stages, the screw unit may be provided only in the first stage. Also in the above embodiments, it is assumed that the cryogenic refrigerator 1, 21 is a GM refrigerator. However, the present invention may be applied to any type of refrigerator including a displacer such as a Stirling refrigerator or a Solvay refrigerator. The above definitions of the top dead point and the bottom dead point may be interchanged with each other.

An aspect of this disclosure provides a cryogenic refrigerator where neither a Scotch yoke mechanism nor a ball screw mechanism is used to cause a displacer to reciprocate, and thereby makes it possible to reduce the size, the number of parts, and the costs of the cryogenic refrigerator.

According to an aspect of this disclosure, a screw unit constituted by a part of a displacer is used to convert the driving force (rotational movement) of a driving unit into reciprocating movement of the displacer. This configuration makes it possible to use a normal motor as the driving unit and to reduce the size, the number of parts, and the costs of a cryogenic refrigerator. 

What is claimed is:
 1. A cryogenic refrigerator, comprising: a cylinder; a displacer configured to reciprocate in the cylinder; a driving unit configured to drive a rotational shaft having an axial direction oriented in a direction of reciprocating movement of the displacer; and a movement conversion unit configured to convert rotational movement of the rotational shaft into the reciprocating movement of the displacer, wherein the movement conversion unit is a screw unit including a male thread and a female thread one of which is a part of the displacer.
 2. The cryogenic refrigerator as claimed in claim 1, wherein the male thread is formed on an outer circumferential surface of the displacer and the female thread is formed on an inner circumferential surface of the cylinder.
 3. The cryogenic refrigerator as claimed in claim 1, wherein the female thread is formed on an inner circumferential surface of a hole extending from a hot-end surface toward a cold-end surface of the displacer.
 4. The cryogenic refrigerator as claimed in claim 3, further comprising: a cylindrical part that is drivably coupled to the rotational shaft such that the cylindrical part is movable in the axial direction relative to the rotational shaft, wherein the male thread is formed on an outer circumferential surface of the cylindrical part.
 5. The cryogenic refrigerator as claimed in claim 2, further comprising: a seal disposed in a clearance formed between the displacer and the cylinder, wherein the seal includes an intermediate layer containing a lubricant and sandwich layers sandwiching the intermediate layer in the axial direction.
 6. The cryogenic refrigerator as claimed in claim 1, further comprising: a rotary valve configured to control intake and exhaust of gas into and from an expansion space formed by the cylinder and the displacer, wherein the driving unit is configured to drive the rotary valve. 